1. Field of the Invention
The present invention relates to a multicolor printing apparatus and, more particularly, to a multicolor ribbon shifting mechanism.
2. Description of the Related Art
A multicolor ribbon having a region coated with yellow ink, a region coated with magenta ink and a region coated with cyan ink arranged in the widthwise direction of the ribbon are typically used as ribbons in multicolor printers. Alternatively, another multicolor ribbon which can be used in multicolor printers has regions coated with yellow, magenta, cyan and black ink, and the like. Each colored region of the ribbon is arranged to be opposed to print paper and is shifted as required by a ribbon shift mechanism which is included in the multicolor printer to control the color being printed
A multicolor dot matrix printer provided with the ribbon shift mechanism described above is disclosed in U.S. Pat. No. 4,568,209. In this printer, the ribbon shift mechanism is driven by a stepping motor so that the region of the multicolor ribbon having a desired color is opposed between the operative portion of the printhead and the print paper. The printer is further provided with a ribbon position detector and a pulse calculator. The ribbon position detector counts the drive pulses supplied to the stepping motor to detect the current position of the ribbon. The pulse calculator calculates the number of drive pulses which must be supplied to the stepping motor in order to perform a ribbon shift operation. The calculator calculates the number of drive pulses based upon the current position of the ribbon as detected by the ribbon position detector and the region of color on the multicolor ribbon to be used for the next print operation. Upon calculation of the number of drive pulses required to be supplied to the stepping motor by the pulse calculator, the calculated number of drive pulses are input into the stepping motor in the same manner irrespective of the magnitude of the drive pulse, and the stepping motor is rotated accordingly. Additionally, in order to maintain a high print speed, the ribbon is shifted while it is in contact with the print paper except during a carriage return operation when the ribbon is located spaced from the print paper.
However, when the ribbon is shifted while it is in contact with the print paper, a number of problems occur. When the ribbon is moved at a high speed during, for example, a large widthwise movement, that is, a large shift amount of the ribbon, the ribbon may be turned over (folded over) due to the friction between the ribbon and the print paper. Particularly, when the ribbon is shifted while it is in contact with seams formed in a continuous paper web, there is a high possibility of the above-described turning-over of the ribbon. Furthermore, when the ribbon is shifted by a large shift amount, ink located on the ribbon is transferred to the print paper at random causing undesirable ink smudges on the print paper. The problems described above occur more frequently as the shift amount of ribbon is larger, for example, when the ribbon is shifted substantially across its entire width.